Why does my phone die when it isn't close to being at 0%?

My phone continuously dies fast and even when it is not dead, it turns off as if it is. Once I plug it into a charger, it will turn back on and will be at the percent that it was on before. Today my phone "died" when it was on 42% and once I plugged it in, it turned on and was at 42%. I don't understand why this is happening and would appreciate it if someone knew how to fix it!